1. Field of the Invention
The present invention relates to a cooling system for a battery (storage battery), which is installed in a vehicle such as a hybrid powered vehicle.
2. Related Arts
A battery used as an electric source for an electric vehicle or a hybrid powered vehicle is required to have high voltage and large capacity. Because of this, the battery is generally composed of a plurality of cells electrically connected with one another in series. This type of battery is referred to as a unit battery.
In the unit battery, however, heat can be developed not only by chemical reactions but also by Joule loss caused during charge and discharge cycles in each cell, resulting in increase in temperature of the unit battery. The increase in temperature advisory affects a battery life and other battery properties. Therefore, various kinds of battery cooling systems are proposed to solve the above problem.
For example, JP-A-8-222280 proposes a heat pipe having an evaporation part at an end thereof and a condensation part at the other end thereof. The evaporation part is inserted into an airtight casing accommodating a plurality of cells therein, while the condensation part protrudes from the airtight casing upward. The heat pipe retains coolant liquid therein. The coolant liquid retained in the evaporation part absorbs heat developed by the cells so that it evaporates and changes into coolant gas. Then the coolant gas rises to the condensation part and loses the heat in the condensation part so that it is condensed to return into liquid state again. At that time, the heat is transmitted from the coolant gas to the outside air.
However, in the unit battery using the heat pipe, there easily arises variation in temperature within the airtight casing. Specifically, in the airtight casing, the cooling effect at a portion apart from the evaporation part of the heat pipe is smaller than that at a portion adjacent to the evaporation part, resulting in the variation in temperature in the airtight casing of the unit battery. If the unit battery has the variation in temperature in the airtight casing, capacities of the cells disposed at a higher temperature part, i.e., at the portion apart from the evaporation part of the heat pipe, are suddenly decreased as the charge and discharge cycles progress compared to those disposed at a lower temperature part. In some kinds of batteries, cells degraded to some extent may generate inflammable hydrogen gas.